Unit 4 DNA: Lesson 3

4.3.1 Mutations reading:

DNA replication does not always make an exact copy of the original DNA molecule. Errors do occur. If a permanent change occurs in a DNA sequence and it can be passed on to an organism's offspring it is called a mutation. A mutation may have no effect on the organism, or if it causes a change in a protein produced during protein synthesis it may have a harmful effect or rarely a beneficial effect on the organism. For example, if a mutation results in a color change, if the organism’s new color makes it better camouflaged the mutation is beneficial. If the mutation makes the organism stand out more so it is more likely to be spotted by predators the mutation is harmful. If the mutation doesn’t cause a change in the organisms traits (phenotype) the mutation has no effect.

There are two categories of mutations. Gene mutations cause changes in a single gene on a chromosome. Chromosomal mutations produce changes in whole chromosomes. Gene mutations that only change one or a few nucleotides are called point mutations and include substitutions, insertions, and deletions.

The picture below shows an example of a substitution mutation.

Missense mutation by U.S. National Library of Medicine / Public Domain

The nucleotide adenine was substituted with the nucleotide cytosine which caused an incorrect amino acid to be added to the protein. As a result the protein may not function properly in the organism. Substitution mutations usually only change one amino acid in the protein. Insertions or deletions result in the addition or removal of a nucleotide and the effects can be severe. These mutations are called frameshift mutations because it can change every amino acid that follows the point of the mutation. Remember bases are read three letters at a time when proteins are produced. If you wrote the sentence “Big cat ate Ted” and took out the third letter like in a deletion mutation the new sentence now reads “Bic ata tet ed”. The sentence does not work just like the protein that is produced will also not function properly.

Chromosomal mutations occur when an organism’s chromosome number or structure is altered which changes the location of genes on the chromosomes or the number of copies of some genes. In deletion mutations all or part of a chromosome is lost. Duplication mutations make extra copies of all or part of a chromosome. Inversion causes a segment of DNA to be flipped which then reattaches to the chromosome. Translocation is when part of one chromosome breaks off and attaches to a completely different chromosome.

Types-of-mutation by National Institutes of Health (NIH)/ Public Domain

Mutagens are chemicals or other agents that can cause mutations to occur more frequently. Chemical mutagens include some pesticides, tobacco smoke, and pollutants. Physical mutagens include UV radiation and x-rays. These substances affect DNA by interfering with base-pairing during DNA replication, or can break the DNA strand causing chromosomal mutations.

DNA_UV_mutation by NASA/David Herring / Public Domain

Mutagens may also sometimes cause cancer.

Cancer_requires_multiple_mutations by the National Cancer Institute / Public Domain

Go on to the next section for a mutations review presentation. You may want to look at the mutations assignment first, because the presentation will help you complete the table.

Sources:

Life Science, CK12 Flexbook http://cafreetextbooks.ck12.org/science/CK12_Life_Science.pdf